Dr. C. Callaicay — On Comparai'we Litliology. 259 



was difficult to meet this attack when it was currently believed that 

 crystalline schists might be of any age. But year after year this 

 belief grew weaker, as mass after, mass of so-called Cambrian or 

 Silurian strata fell back into the Archeean ; until at last those who 

 had made a special study of the old rocks thought they could detect 

 amidst their complexities the dim outline of a law. The existence 

 of this principle, if it can be established, will raise comparative 

 lithology into a valuable test of the age of a rock-system. I will 

 not venture to definitely formulate this suggestion ; but I will go so 

 far as to say that our present knowledge lends some support to the 

 conclusions that, in the British area at least, crystalline schists have 

 not been manufactured on a large scale in Post-Archeean times, and 

 that, amongst the Archaean rocks, the antiquity of a schist is in 

 direct ratio to its degree of crystallization. I do not say "degree of 

 alteration," because this would involve a theory, and introduce com- 

 plication. 



I have not forgotten Prof. Lapworth's ingenious application of the 

 mechanico-chemical hypothesis of metamorphism to certain of the 

 Highland schists. But as he has, with rare magnanimity, abandoned 

 the publication of the evidence to the hands of the Geological 

 Survey, we must postpone discussion of the question till the official 

 memoirs are issued. Whatever the result, the main conclusions of 

 Archaean geologists will not be materially affected. Even if all the 

 newer Highland schists were thrust back into the Silurian system, 

 the Archaean age of the schists of England and Wales would not be 

 touched. Each case has been established by independent evidence, 

 and can be overthrown only by the refutation of that proof. But 

 should ray Caledonian system be destroyed by the new theory, some 

 modification of the suggested law would of course be necessary. It 

 would still be applicable within certain limits and under certain con- 

 ditions. Indeed, I do not even now claim for it any more than an 

 empirical and local value. 



I start with the proposition that in Britain there occur (at least) 

 two Archaean groups, of which the older is coarsely crystalline, and 

 the younger either eruptive or hypocrystalline. These are the 

 Hebridean and the Pebidian. There is some reason for believing in 

 the existence of a third series, intermediate in age and degree of 

 crystallization between the two ; but to limit controversy I exclude 

 this group. The Hebridean and the Pebidian are as truly rock- 

 systems as the Cambrian and the Silurian. I will briefly review 

 the evidence. 



At St. Davids, we have a granitoid mass (Dimetian) overlain by a 

 newer group, partly volcanic, partly hypocrystallnie, constituting the 

 typical Pebidian of Hicks. The Archasan age of the Dimetian has 

 been recently attacked, so that I will not insist upon it here ; but it 

 is admitted on all hands that the Pebidian underlies rocks containing 

 the most ancient Cambrian fossils known ; so that even if there were 

 no break at the base of the fossiliferous beds, we must grant that the 

 Pebidian is more ancient than anything which can be proved to 

 be Cambrian. 



